Glass is widely used as a transparency in a variety of applications due to its superior optical qualities. For example, glass is commonly used as glazing material or as an architectural material for buildings. Glass is also commonly used as a transparency in a variety of vehicular applications. Unfortunately, glass is a relatively dense material and is also relatively brittle such that relatively large thicknesses are required to provide the glass with sufficient strength to resist shattering when impacted by an object.
In an attempt to avoid the weight penalties associated with glass, transparencies may also be fabricated of polymeric material. For example, transparencies may be formed of transparent polymers such as acrylic (e.g., Plexiglas™) which is less dense than glass and which possesses suitable optical properties. Unfortunately, acrylic has relatively low strength properties making it unsuitable for many applications where high impact resistance is required.
In consideration of the weight penalties associated with glass and the strength limitations associated with transparent polymers, manufacturers have fabricated transparencies from polymeric materials reinforced with glass fibers to enhance the strength and impact resistance of the polymeric transparency. Unfortunately, the addition of glass fibers to polymeric material may undesirably affect the optical quality of the transparency. For example, the glass fibers may have a cylindrical configuration such that each one of the glass fibers acts as a small lens. The effect of a plurality of the glass fibers, each acting as a small lens, is a scattering of light as the light passes through the transparency such that objects viewed through the transparency may appear blurred.
A further drawback associated with transparencies fabricated from glass fiber-reinforced polymeric materials is the variation in the refractive indices of the glass material and polymeric material as temperature changes. Refractive index, represented by n(λ,T), is a function of wavelength λ incident on a material at temperature T. In the case of glass fiber-reinforced polymeric materials, the refractive index of the polymeric material generally decreases with increasing temperature for a given wavelength or wavelength band such as the visible spectrum. In contrast, the refractive index of glass typically varies only slightly with changes in temperature for the visible spectrum.
Such a change in refractive index of a material with temperature change of a material for a given wavelength may also be defined as the temperature coefficient of refractive index of the material, dn(λ,T)/dT. In the expression dn(λ,T)/dT, do represents the change in refractive index of the material, λ represents the wavelength of radiation (e.g., light) incident on the material, T represents temperature, and dT represents the change in temperature of the material. It should be noted that although a material may be described in terms of its refractive index at one or more wavelengths and temperatures, the temperature coefficient of refractive index of a material is also typically listed with the refractive index data for the material.
Although glass and polymeric material may be selected to have the same refractive index at a given match point temperature for a given wavelength, the differences in temperature coefficient of refractive index dn(λ,T)/dT of the glass as compared to the temperature coefficient of refractive index dn(λ,T)/dT of the polymeric material results in a change (e.g., an increasing difference) in the refractive indices of the two materials as the temperature diverges from the match point temperature. The change in refractive indices of the glass and polymeric material as temperature changes may result in a corresponding reduction in optical quality of the transparency with change in temperature due to scattering of light at the glass/polymer interface.
As can be seen, there exists a need in the art for an optically transparent composite article which has a relatively high degree of optical transparency with minimal optical distortion within a relatively broad temperature range and which exhibits improved ballistic and mechanical performance with minimal weight.